1. Field of the Invention
The present invention is directed generally to a rapid-hardening hydraulic cement composition and related products. More particularly, the present invention is directed to a rapid hardening hydraulic cement composition comprising subbituminous fly ash and having no portland cement.
2. Technical Background
Electrical generation stations throughout the world burn one or more types of coal to produce electricity. One by-product of coal combustion is an inorganic residue known as “fly ash” or “pulverized fuel ash.” Individual ash particles are very small (like face powder), and are carried up and out with the flow of combustion exhaust gases; hence the term “fly” ash. Thus, fly ash has been defined as the finely divided mineral residue resulting from the combustion of ground or powdered coal.
Fly ash is a “pozzolan”, i.e., a material containing silica, alumina and calcium that, in the presence of water, will chemically combine with free lime contained in either the fly ash or other components to produce a material with cementious properties. Some fly ashes contain sufficient calcium compounds to be self-hardening, while others require the addition of calcium (usually in the form of cement or lime) to harden. There are two main types of fly ash: Class F (low lime) and Class C (high lime). Class F fly ash contains silica, aluminum and iron in combinations of greater than 70% and possesses little or no cementitious value when used alone but will, under appropriate conditions, chemically react with calcium hydroxide at ordinary temperatures to form cementitious compounds. Class F fly ash is typically associated with eastern and midwestern U.S. coals, such as anthracite or bituminous coal. In comparison, Class C fly ash generally contains more calcium and less iron than Class F fly ash, and has a lime (CaO) content in the range of 15% to 30%. Thus, Class C fly ash is cementious or self-hardening. Class C Fly ash is usually associated with western U.S. coals, such as lignite or subbituminous coal.
Fly ash is collected from the combustion gases using a variety of means, such as electrostatic precipitators, FGD systems or baghouses. Not surprisingly, fly ash accumulates rapidly and can cause enormous waste disposal problems. Consequently, there has been a tremendous effort to find suitable recycling uses for fly ash as alternatives to landfilling.
Historically, fly ash has been used as a strengthening additive in portland cement. Portland cement requires great amounts of energy for manufacture and generally speaking, for each ton of portland cement produced, one ton of carbon dioxide is emitted. Therefore, the use of fly ash as a replacement for all or some of the portland cement leads to the useful utilization of a landfill material as well as a reduction in the energy and CO2 emissions associated with portland cement production. Using fly ash in this manner bestows a great benefit to society in the form of resource conservation and environmental protection.
The building materials industry is a leader in utilizing fly ash in cement and concrete. For example, fly ash is disclosed for use in cement compositions in U.S. Pat. No. 4,842,649 to Heitzmann et al. The composition contains from 50 parts to about 80 parts portland cement; from 13 parts to about 35 parts fly ash; and from 1 part to about 5 parts potassium carbonate. The composition may additionally include up to about 10 parts metakaolin; up to about 6 parts slag; and up to 4 parts of an admixture.
U.S. Pat. No. 4,997,484 to Gravitt et al. discloses a cement that uses 80.8 to 99 parts of Class C fly ash to achieve high strength in a short time, usually less than thirty minutes, when cured at room temperature.
U.S. Pat. No. 5,997,632, filed by the applicant herein, discloses blended hydraulic cement that comprises about 85 to 99.7 weight percent subbituminous fly ash. The full and complete disclosure of that patent is incorporated herein by reference for all purposes as if fully set forth.
U.S. patent application Ser. No. 10/249,935, filed May 20, 2003, discloses blended hydraulic cement that comprises vitrified fly ash blended with portland cement or lime.
The inventions disclosed and claimed herein advance and improve the technology set forth above in the form of a rapid-hardening hydraulic cement made for fly ash that utilizes no portland cement.